Many publications describe means to monitor the residual charge in batteries and in particular automotive batteries, where the amount of charge remaining may determine whether or not the battery can be used at a later time to drive a starter motor and effectively start an engine.
Also, a discharged lead-acid battery will rapidly deteriorate (by a process known as sulphation). There are many other applications of batteries where it is desirable to know the amount of residual charge in the battery, for example batteries used for cell phones, or for emergency equipment. In the case of nickel-cadmium ("nicad") batteries it is desirable to prevent recharging unless the battery is fully discharged or has less than (say) 10% of maximum charge.
Although modern motor vehicle batteries have progressively improved in capacity there remains a risk that the battery can be accidentally discharged, for example, by leaving the lights on when leaving the vehicle or in other similar ways. Thus the driver on returning to the vehicle will not be able to start the engine because insufficient voltage or capacity remains in the battery to turn the starter motor or to provide sufficient power to the spark plugs, for example, to provide a useable spark. This can of course extremely inconvenient for the driver. In some situations, for example, in extreme weather conditions or in localities offering high personal risk a flat battery can be actually dangerous.
Some attempts have been made to overcome this problem, for example, by providing what are in effect double batteries or pairs of batteries but such arrangements may not solve the problem in all circumstances as an opportunity for the starting battery or for the vehicle starting cells to go flat, still remains.
Paredes (U.S. Pat. No. 4,020,414) is one of many inventions that uses battery voltage during discharge as a gauge of capacity, to warn the user of potential failure. These are in effect narrow-range voltmeters for the 11-12 V range, and assume use with a 12 V battery. Paredes uses an array of lamps as a voltage display. Paredes teaches (as in his FIG. 3) that there is a gradual decline of cell voltage as the level of charge is reduced. Stevens (U.S. Pat. No. 4,028,616) includes with his voltmeter a method for measuring voltage under no load and then after a ten-second engine start routine, in order to assess battery condition.
During research for this invention and that of my earlier PCT/NZ96/00060, the problem of relying solely on voltage as per the above prior art was realised. It gives too inaccurate a cutoff point.
Sheldrake (U.S. Pat. No. 4,493,001) teaches a motor battery rundown protection system which does include disconnection and reconnection means but this system has only voltage sensing means. The same can be said for Sloan (U.S. Pat. Nos. 4,902,956 and 5,332,958).
There exist many methods for determining the remaining charge in a battery but their complexity makes them potentially unreliable and too expensive to achieve universal acceptability.
The use of a microprocessor to store a look up table as described in our earlier PCT/NZ96/00060 results in an effective but expensive solution which is suited to some applications, e.g. monitoring truck batteries, or the batteries used on heavy machinery. There is a need for a less expensive solution.
Definitions
"A critical load" is a term used to refer to an important use of battery charge, such as for turning over an internal-combustion engine as part of an engine starting process.
"A non-critical load" is a term used to refer to a less important and possibly inadvertent use of battery charge, such as for running headlights, parking lights, or a radio.
"A defined type of battery" refers to a set of batteries having substantially similar voltage/current/state-of-charge characteristics, and for example refers in general to the class of conventional lead-acid storage batteries as are used in petrol or diesel-driven cars and similar road vehicles. (Some individual products may differ in characteristics from the majority; perhaps due to different details of design). Sealed, gel-electrolyte lead-acid storage batteries may comprise a different set of known battery types and nickel-cadmium rechargeable batteries are likely to comprise a different set.
Object
The object of this invention is to provide an improved method for determining whether or not the remaining charge in a battery has fallen below some predetermined threshold.
Statement of Invention
In one aspect the invention provides a battery charge monitoring means for deriving an indication of the state of charge of a defined type of battery, the monitoring means having:
means capable of sensing a load voltage proportional to current flowing when a defined type of battery is connected to a load, PA0 a fixed reference voltage which is less than the maximum voltage of the defined type of battery to which the monitoring means is to be connected, PA0 current sensing means capable of providing a voltage output equivalent to the voltage drop within the defined type of battery when under load as a result of the internal resistance of the defined type of battery, PA0 means capable of sensing a dynamic reference voltage comprising the fixed reference PA0 voltage minus the voltage output of the current sensing means, PA0 comparison means capable of comparing (a) the load voltage and (b) the dynamic reference voltage to provide an output indicative of the state of charge of the battery,
Preferably the means is connected to evaluation means capable of determining the state of charge of the battery based on an output from the comparison means.
Preferably the comparison means comprises a comparator.
Preferably the current sensing means comprises a current sensing resistor having a resistance of the same order of magnitude as the internal resistance of the defined type of battery to which the monitoring means is to be connected.
Preferably the current sensing means comprises a Hall effect sensor.
Preferably the fixed reference voltage for a 12 volt 40 ampere-hour lead acid battery is about 25 volts.
Preferably the monitoring means is connected to an automotive battery between a non-critical load and one of the battery terminals and includes at least one switching means in series with the non-critical load capable of connecting or disconnecting the non-critical load from the automotive battery, the switching means being controlled by an output from the comparison means.
Preferably the evaluation means comprises a microprocessor connected to the output from the comparison means, said microprocessor being capable of controlling the switching means.
Preferably the microprocessor is capable of controlling in use a visual display means to signal if the battery state of charge is above or below a predetermined threshold value.
Preferably the monitoring means includes means for sensing a change in the non-critical load to cause the microprocessor to control the switching means to reconnect the non-critical load to the battery when the microprocessor receives a signal that a change in the non-critical load has been detected.